This paper presents some structures of artificial coplanar waveguide with very slow phase velocity and their applications to a design of compact 3-dB branch-line couplers. The slow-wave structure is constructed by periodically loading both of series inductance and shunt capacitance. First, a basic miniature branch-line coupler is designed and consequently considerable size-reduction of about 1/4 is obtained. Next, a broadband design technique is described using open-circuited quarter-wavelength series-stubs added at each port as a matching network. By size-reducing the series-stubs and branchline sections, a very compact broadband coupler with a good hybrid performance over a wide bandwidth of 31 percent or more is realized. The design concepts and procedures are verified both numerically and experimentally.

This paper presents a new quarter-wavelength microstrip coupler compensated with a periodic sequence of floating metallic strips in the slots on the inner edges. After describing the characteristics of the coupled-line, as an example, a 15-dB coupler is designed and a high directivity of 30 dB or more in theory is obtained over a full band of a single-section coupler. Next, couplers with various coupling factors are designed, and the usefulness for very loose coupling is demonstrated. Furthermore, a three-section coupler is designed to show the effectiveness in a wide frequency range. The validity of the design concept and procedure is confirmed by electromagnetic simulations and experiments.